The present invention relates generally to atmospheric fluidized bed combustion, pressurized fluid bed combustion, fluidized bed coal gasification, or any other fluid bed application, and more particularly to a distribution system for uniformly feeding coal into the reaction chamber of the fluidized bed.
The ever growing demand for energy has greatly increased the reliance upon the combustion of coal for the generation of electrical power. The utilization of coal in combustion systems as previously known has not proven to be particularly efficient and also presented a considerable environmental problem due to the substantial quantities of polluting emissions from the stacks of various utility plants. The shortage of clean coal, i.e., low-sulfur coal, makes it increasingly difficult for the conventional furnace designs to provide adequate combustion of coal while maintaining pollutants within environmental limits without the utilization of expensive precipitators and other pollutant removal equipment.
A fairly recent improvement in coal combustion systems is in fluidized bed combustion processes which possess the capability of controlling particulate emissions as well as sulfur dioxide and NO.sub.X within the limits set by the Environmental Protection Agency while burning virtually any coal or low-grade fuels available in the United States. The fluidized bed combustion process has proven to be a highly efficient mechanism for extracting a significant percentage of available Btu values from the coal and converting this heat energy into useful products such as steam for use in a power generating plant.
A fluidized bed process is generally described as being provided by a bed of granular particulates initially supported in a combustion zone by a perforated grid through which air may be passed with sufficient velocity to suspend the particulates in a constantly moving "fluid-like" manner in the combustion zone above the grid. Generally a noncombustible material, such as limestone or dolomite, is used to provide the bed particulates. Coal particulates are introduced into the constantly stirred bed and are heated to a temperature sufficient to support the combustion of the coal. The temperature of the bed and the combustion process can be readily controlled by regulating the flow of air through the grid and the volume of coal. Also, the particular heat transfer surfaces, e.g., steam generating coils and the like, disposed in the combustion zone may be used to control the temperature of the combustion process. By employing limestone bed material in a combustion zone, a substantial percentage of the sulfur dioxides released during the combustion of the coal react with and are absorbed by the limestone. The nitrous oxides (NO.sub.X) are maintained at a relatively low formation level due to the low combustion chamber temperature of about 1500.degree. to 1600.degree. F. required for practicing the fluidized combustion process.
It is believed that fluidized combustion processes will be particularly useful for steam generation and are expected to be of significant value in helping to overcome the energy problems that we are presently facing. One of the difficulties associated with the fluidized bed combustion process is in the techniques utilized for the introduction of the coal into the fluidized bed. For example, the practices presently employed for introducing coal into the fluidized bed require a plurality of conduits or pipes of dissimilar lengths extending into the fluidized bed to different locations so that the coal distribution can be provided with some degree of uniformity. However, even with such an extensive conduit network within the fluidized bed, it has been found that the uniformity of the coal distribution is insufficient for achieving optimum efficiency from the fluidized bed operation. Screw-type conveyors have not proven practical due to a lack of uniformity in distribution of the feed in the bed and the relatively low back pressure at which the conveyors will function. Also, with known coal feeding systems variations in moisture and feed size have presented significant problems often resulting in total shutdown of the fluidized bed.
Accordingly, it is the primary aim, or goal, of the present invention to provide apparatus for feeding coal into a fluidized bed combustor with a more uniform distribution than previously attainable. The coal-feeding means of the present invention is utilized in the fluidized bed combustor which is typically of an elongate vertically oriented configuration with an air inlet at the lower end of the combustion chamber and a discharge or exhaust at the upper end of the combustion chamber. The coal-feeding means of the present invention transports coal into the combustion chamber at a location intermediate the air inlet and the discharge and comprises a movable conveying means which is transversely disposed within the combustion chamber. Coal supply means are placed in registry with the conveying means at a location adjacent one end of the conveying means for delivering to the latter a preselected quantity of coal. Drive means are, in turn, coupled to the conveyor means for driving the conveyor means and transporting the coal placed thereon into the combustion chamber. The conveyor means is characterized by being provided with a plurality of substantially uniformly spaced-apart passageways extending therethrough so that air from the air inlet will pass through these passageways in the conveyor means and displace the coal from the conveyor means into the combustion chamber above the conveying means. This coal-feeding means of the present invention provides the introduction of the coal at the bottom of the fluidized bed with a substantially uniform distribution of the coal across the entire cross section of the combustion chamber and also provides a maximum residence time of the coal within the bed so as to facilitate the control of the coal feed.
Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.
A preferred embodiment of the invention has been chosen for the purpose of illustration and description. The preferred embodiment illustrated is not intended to be exhaustive or to limit the invention to the precise form disclosed. It is chosen and described in order to best explain the principles of the invention and their application in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications as are best adapted to the particular use contemplated.